A toroidal type continuously variable transmission for use with a vehicle is disclosed, for example, in Tokkai Hei 3-89066 published by the Japanese Patent Office in 1991. In this controller, a pair of power rollers are gripped between input/output cone disks on the same rotation axis, and a speed change ratio is continuously varied according to a tilt of these power rollers.
The power rollers are supported by a trunnion, and their inclination is varied by displacing the trunnion in specified directions respectively at right angles to the rotation axes of the rollers and the disks. Such an assembly is disclosed for example in SAE Technical Paper No. 901761 published by the Engineering Society For Advancing Mobility in 1990.
Each trunnion is associated with an oil pressure piston, and hydraulic pressure is supplied to the oil pressure piston via a control valve comprising a spool and a sleeve linked to a stepping motor. The stepping motor rotates according to a speed change command value or a step number input from a controller comprising a microprocessor.
An offset amount from a neutral position of the trunnion in the aforesaid specified direction is mechanically fed back to the spool of the control valve via a cam or a link. Due to this feedback, the trunnion offset amount is controlled so as to obtain a power roller gyration angle depending on the command value.
As the offset of the trunnion is only of the order of several millimeters, when part of the trunnion deforms due to the transmitting torque of the power roller, an error arises between a target speed change ratio and a real speed change ratio.
One way of reducing this error would be to use an electronic feedback system wherein the real speed change ratio is measured using a sensor, and the controller controls the stepping motor such that the real speed change ratio coincides with the target speed change ratio.
In this electronic feedback system, it is desirable to perform PID (Proportional Integral Differential) control according to the characteristics of a speed change target value depending on engine running conditions.
In PID control, when a discrepancy arises between a target value and a measured value, a direct proportional output P is first generated, and then an integral control output I, which is an integral value, is generated.
In this case, when the variation of the discrepancy exceeds a stepping motor operating speed limit (response speed limit), a target step number given to the stepping motor and real step number accomplished by the stepping motor do not coincide, and as a result the discrepancy is not eliminated. Hence the direct proportional output P does not disappear, and an integral cumulative value of an integral control output value also increases.
At some point, when the real speed change ratio coincides with the target speed change ratio, the direct proportional control output P disappears, however the integral control output I still remains due to the drive speed lit of the step motor and it continues to accumulate. As a result, the real speed change ratio overshoots the target speed change ratio. In a vehicle transmission, this overshoot of the speed change ratio causes the drivability of the vehicle to be impaired.